Modules

Introduction

As programs grow, keeping everything in a single file becomes messy and hard to maintain. Python solves this with modules — a simple way to split code across multiple files and reuse it wherever it is needed.

In this lesson, you will learn exactly what a module is, the different ways to import one, a very important idiom every Python script should know, and how Python actually locates the modules you import.

What is a Module?

A module is simply any Python file with a .py extension. That's the whole definition — every .py file you write is already a module the moment it exists. Modules let you organize related functions, classes, and variables into separate files, and then reuse that code by importing it elsewhere.

Creating Your Own Module

Imagine you have a file named greetings.py containing a couple of reusable functions.

def say_hello(name):
    return f"Hello, {name}!"

def say_goodbye(name):
    return f"Goodbye, {name}!"

That file is now a module named greetings. As long as another Python file is in the same folder (or Python can otherwise locate it), it can import and use these functions.

import greetings

print(greetings.say_hello("Maya"))
print(greetings.say_goodbye("Maya"))

Hello, Maya!
Goodbye, Maya!

import module

The most common way to use a module is a plain import statement. This makes the entire module available, and you access its contents using dot notation: module.name.

import math

print(math.sqrt(16))
print(math.pi)

4.0
3.141592653589793

Everything inside the math module is accessed through the math. prefix, which keeps it clear exactly where each function or value came from.

from module import name

If you only need one or two specific things from a module, you can import them directly by name, without needing the module prefix afterward.

from math import sqrt, pi

print(sqrt(25))
print(pi)

5.0
3.141592653589793

You can also import everything from a module using from module import *, but this is generally discouraged because it makes it unclear where each name came from, and it can silently overwrite names already in your file.

import module as alias

Sometimes a module name is long, or clashes with something else you are using. You can give it a shorter alias with the as keyword.

import datetime as dt

today = dt.date.today()
print(today)

2026-07-25

Aliasing is extremely common with certain well-known third-party libraries, such as import pandas as pd or import numpy as np, which you will encounter in later, more advanced lessons.

The if __name__ == "__main__": Idiom

Every Python module has a built-in variable called __name__. When a file is run directly, Python sets __name__ to "__main__". When that same file is imported by another file instead, __name__ is set to the module's actual name.

def add(a, b):
    return a + b

print(f"This module's __name__ is: {__name__}")

if __name__ == "__main__":
    print("Running calculator.py directly")
    print(add(2, 3))

This module's __name__ is: __main__
Running calculator.py directly
5

import calculator

print(calculator.add(10, 20))

This module's __name__ is: calculator
30

Notice that the "Running calculator.py directly" line never printed in the second example. This idiom lets a file contain test code or a demo that only runs when the file is executed directly — not when it is imported as a module elsewhere.

Built-in Modules

Python ships with a large standard library of built-in modules that are always available, with no installation required. You have already seen math and datetime above — here is a quick look at a few commonly used ones. Each gets its own full lesson later in this course.

import random

print(random.randint(1, 6))   # simulate a dice roll

4

How Python Finds Modules

When you write import something, Python needs to locate the actual file. It searches a list of locations stored in sys.path, in order, and uses the first matching module it finds.

import sys

for location in sys.path:
    print(location)

C:\Users\you\project
C:\Python312\Lib
C:\Python312\Lib\site-packages

This is why a module you create in the same folder as your script can be imported immediately — that folder is automatically part of the search path.

Common Mistakes

Best Practices

Frequently Asked Questions

Key Takeaways

Summary

Modules are the basic unit of code reuse in Python — any .py file can be written once and imported wherever its functions or classes are needed. Understanding import, from...import, and import...as gives you full control over exactly what gets brought into your file.

You also saw why almost every real Python script uses the if __name__ == "__main__": idiom, and how Python's sys.path determines which modules can actually be found.